Due to recent advances in biotechnology, therapeutic proteins and other biomolecules, e.g. antibodies and antibody fragments, can now be prepared on a large scale, making such biomolecules more widely available. Unfortunately, the clinical usefulness of potential therapeutic biomolecules is often hampered by their rapid proteolytic degradation, low bioavailability, instability upon manufacture, storage or administration, or by their immunogenicity. Due to the continued interest in administering proteins and other biomolecules for therapeutic use, various approaches to overcoming these deficiencies have been explored.
One approach that has been widely explored is the modification of proteins and other potentially therapeutic molecules by covalent attachment of a water-soluble polymer such as polyethylene glycol or “PEG” (Abuchowski, A., et al, J. Biol.Chem. 252 (11), 3579 (1977); Davis, S., et al., Clin.Exp Immunol., 46, 649-652 (1981). The biological properties of PEG-modified proteins, also referred to as PEG-conjugates or pegylated proteins, have been shown, in many cases, to be considerably improved over those of their non-pegylated counterparts (Herman, et al., Macromol. Chem. Phys., 195, 203-209 (1994). Polyethylene glycol-modified proteins have been shown to possess longer circulatory times in the body due to increased resistance to proteolytic degradation, and also to possess increased thermostability (Abuchowski, A., et al., J. Biol. Chem., 252, 3582-3586 (1977). A similar increase in bioefficacy is observed with other biomolecules, e.g. antibodies and antibody fragments (Chapman, A., Adv. Drug Del. Rev. 54, 531-545 (2002)).
Typically, attachment of polyethylene glycol to a drug or other surface is accomplished using an activated PEG derivative, that is to say, a PEG having at least one activated terminus suitable for reaction with a nucleophilic center of a biomolecule (e.g., lysine, cysteine and similar residues of proteins). Most commonly employed are methods based upon the reaction of an activated PEG with protein amino groups, such as those present in the lysine side chains of proteins. Polyethylene glycol having activated end groups suitable for reaction with the amino groups of proteins include PEG-aldehydes (Harris, J. M., Herati, R. S., Polym Prepr. (Am. Chem. Soc., Div. Polym. Chem), 32(1), 154-155 (1991), mixed anhydrides, N-hydroxysuccinimide esters, carbonylimadazolides, and chlorocyanurates (Herman, S., et al., Macromol. Chem. Phys. 195, 203-209 (1994)). Although many proteins have been shown to retain activity during PEG modification, in some instances, polymer attachment through protein amino groups can be undesirable, such as when derivatization of specific lysine residues inactivates the protein (Suzuki, T., et al., Biochimica et Biophysica Acta 788, 248-255 (1984)). Moreover, since most proteins possess several available/accessible amino groups, the polymer conjugates formed are typically mixtures of mono-pegylated, di-pegylated, tri-pegylated species and so on, which can be difficult and also time-consuming to characterize and separate. Further, such mixtures are often not reproducibly prepared, which can create problems during scale-up for regulatory approval and subsequent commercialization.
One method for avoiding these problems is to employ a site-selective polymer reagent that targets functional groups other than amines. One particularly attractive target is the thiol group, which in proteins in present in the amino acid, cysteine. Cysteines are typically less abundant in proteins than lysines, thus reducing the likelihood of protein deactivation upon conjugation to these thiol-containing amino acids. Moreoever, conjugation to cysteine sites can often be carried out in a well-defined manner, leading to the formation of single species polymer-conjugates.
Polyethylene glycol derivatives having a thiol-selective reactive end group include maleimides, vinyl sulfones, iodoacetamides, thiols, and disulfides, with maleimides being the most popular. These derivatives have all been used for coupling to the cysteine side chains of proteins (Zalipsky, S. Bioconjug. Chem. 6, 150-165 (1995); Greenwald, R. B. et al. Crit. Rev. Ther. Drug Carrier Syst. 17, 101-161 (2000); Herman, S., et al., Macromol. Chem. Phys. 195, 203-209 (1994)). However, many of these reagents have not been widely exploited due to the difficulty in their synthesis and purification.
Polyethylene glycol derivatives having a terminal maleimide group are one of the most popular types of sulfhydryl-selective reagents, and are commercially available from a number of sources. Although not widely appreciated or recognized, the Applicants have recognized that many PEG-maleimides unfortunately exhibit hydrolytic instability during storage and/or conjugation to a drug candidate. More particularly, a substantial degree of hydrolysis of the maleimide ring has been observed, both prior to and after conjugation. This instability can result in the formation of multiple species of drug conjugates within a drug-conjugate composition. The various drug conjugate species are likely to possess similar biological activities, but may differ in their pharmacokinetic properties. This is particularly disadvantageous for compositions intended for patient administration, since the resulting drug compositions can be ill-defined mixtures of drug conjugate species whose particular safety and accumulation profiles are unknown. Moreover, due to different factors impacting hydrolysis rates, inconsistency between drug conjugate batch compositions can present an additional problem.
Another potential problem that has been observed by the applicants is the de-pegylation of conjugates prepared from PEG maleimides to yield mixtures of altered drug and detached PEG impurity. For these reasons, the Applicants have found that PEG maleimides can be undesirable reagents for coupling to thiol groups on target drugs or other active agents. Previous attempts to address this problem have focused on increasing the stability of a polymer maleimide by making it less prone to hydrolysis (i.e., ring-opening). See for example, U.S. Patent Application Publication No. US 2003/0065134.
Thus, the applicants have realized a continuing need in the art for the development of new activated PEGs useful for coupling to biologically active molecules, desirably in a site-selective fashion, that overcome the shortcomings of presently-available thiol-selective polymer reagents and are stable during both storage and coupling. This invention meets those needs.